Blood is circulated through the body by the contraction of the heart. Referring to FIG. 1, which illustrates a blood pressure wave obtained by an invasive technique, when the heart contracts it pumps blood into the arteries. This generates, typically, a maximum pressure of 120 mm Hg, which is referred to as the systolic pressure. When the contraction terminates, the pressure falls, typically, to 80 mm Hg, which is referred to as the diastolic pressure. The dashed line, designated A, generally indicates the significantly reduced pressure waveform that would be measured at the surface of the skin, as opposed to within the artery itself.
The mean blood pressure, often referred to as the Mean Arterial Pressure (MAP), is an average blood pressure over the entire contraction cycle. The MAP is, however, not the average of 120 and 80, in that the blood pressure is nearer to the diastolic pressure during most of the contraction cycle. As a result, the MAP is typically about 90 mm. Hg. The determination of the MAP is considered to be important, in that it generally provides more information concerning the overall functioning of the subject's circulatory system than do measurements of only the systolic and diastolic pressures.
The systolic and diastolic pressures are typically measured by wrapping a blood pressure cuff around an extremity containing an artery, inflating the cuff to a predetermined pressure above the systolic pressure, thereby occluding the artery, and reducing the pressure gradually allowing increased flow through the artery. Oscillations resulting from arterial blood flow through the partially occluded artery are sensed in some manner for an entire blood pressure measuring cycle.
The most common method for measuring blood pressure indirectly is with a sphygmomanometer and stethoscope. Advantages of the sphygmomanometer and stethoscope include simplicity and low cost. However, the procedure is relatively inaccurate, and produces only a limited amount of diagnostic information.
The procedure by which a sphygmomanometer is utilized to determine blood pressure is relatively simple. A collapsed, inflatable blood pressure cuff is first placed around a subject's arm. Pressure in the cuff is then increased to a level of about 30 mm. Hg. above a point at which the palpable pulse disappears. As the cuff is deflated, observations may be made either by palpation or auscultation. The point at which the pulse can be felt is recorded from the manometer as the systolic pressure.
The auscultatory method is usually preferred to the technique described above. With this method, vibrations from the artery under pressure, called Korotkoff sounds, are used as indicators.
To determine blood pressure using the auscultatory method, the bell or diaphragm of a stethoscope is pressed lightly over a brachial artery while the cuff is slowly deflated. The pressure readings begin at the time the Korotkoff sounds first become audible. As the cuff is deflated further, the sounds become louder for a brief period. The sounds then become muffled and finally disappear. The systolic blood pressure is the point at which the Korotkoff sounds become audible, and the diastolic blood pressure is the point at which the sounds cease to be heard.
There are several other known methods for accomplishing the measuring of the systolic and the diastolic pressures. Two conventional methods, that rely on the use of electronic components, are known in the art as oscillometry and tonometry.
In oscillometry, the oscillations resulting from arterial blood flow through the partially occluded artery are transmitted as pressure variations through the cuff and tubings to a sensitive pressure transducer located within a monitor. However, in that the pressure transducer must be highly sensitive, the slightest movement of the subject can give rise artifacts which reduce the accuracy of the measurement. As a result, artifact detection and rejection circuits may be required to be included within the monitor. Furthermore, the oscillometric method typically requires a stepwise reduction of cuff pressure. In addition, blood pressure measurement may not be possible at all if the subject's blood pressure is below a threshold value.
In tonometry a transducer is placed directly over a superficial artery, such as the radial artery, and is compressed against an underlying bone with a predetermined pressure. Oscillations resulting from arterial blood flow through the artery are converted to electrical signals by the transducer and are subsequently detected. In practice, the transducer is smaller than the diameter of the artery. As a result, the tonometric sensor must be positioned very accurately over the artery, and must also partially compress the artery. As a result, even a small misalignment makes it difficult or impossible to accurately measure blood pressure. Furthermore, accurate calibration of the tonometric system is typically necessary.
U.S Pat. No. 4,427,013 describes the use of a determination of an increase and a decrease in the rate of a rise and a fall, respectively, of a blood pressure waveform to determine the systolic and the diastolic pressures. In U.S. Pat. No. 4,860,760 there is described the use of a first fixed threshold, referenced to a peak blood pressure, to indicate the systolic pressure, and a second fixed threshold to indicate the diastolic pressure.
These and other U.S. Patents of interest to the teaching of this invention are described below.
U.S. Pat. No. 5,031,630, issued Jul. 16, 1991, entitled Automatic Blood Pressure Measuring Apparatus, describes a cuff 10 having six piezoelectric sheets 18-28 disposed on a cuff surface that contacts the body of the patient. Sheets 18-22 are arranged to detect proximal artery sounds, while sheets 24-28 are located to detect Korotkoff sounds transmitted from the artery to a middle area of the cuff (col. 3, line 48-col. 4, line 3). Signals generated by the sheets 18-28 are digitized and applied to a CPU 34, as is the output of a cuff pressure sensor 36 (col. 4, lines 4-24). The sheets (18-22) are monitored to select an optimum sensing sheet, after which the sheet (24-26) that is aligned with the selected sheet (18-22) is also selected (col. 5, lines 14-44). The selected piezoelectric sheets are used to monitor Korotkoff sounds, which are used in conjunction with cuff pressure to determine blood pressure (col. 6, lines 4-48). The size of the individual (cuff-mounted) piezoelectric sheets is not disclosed (i.e., whether they are smaller or larger than the artery).
U.S. Pat. No. 5,165,416, issued Dec. 24, 1992, entitled Continuous Blood Pressure Monitoring System Having a Digital Cuff Calibration System and Method", describes the use of a tonometric sensor 10 and a digital cuff 20. Both the sensor 10 and cuff 20 receive a pressurized fluid (col. 2, lines 53-54). The cuff 20 is positioned downstream of sensor 10 (col. 4, lines 38-42). A calibration sensor 22 can be added to the cuff 20. The sensor 22 can be a microphone for detecting Korotkoff sounds, or may be an optical sensor (col. 4, lines 47-57)
U.S. Pat. No. 4,880,013, issued Dec. 14, 1989, entitled "Method and Apparatus For Determining Blood Pressure and Cardiovascular Condition" describes the use of a pressure transducer 16 and a cuff 10. Both are said to be affixed to the patent (col. 4, lines 36-37). A data stream is obtained from the pressure transducer 16, the data stream including both pressure data and pulsation signal data. Systolic maximum points and diastolic minimum points are determined. Two methods of determining Mean Arterial Pressure (MAP) are also disclosed; specifically a "midpoint method" and a "mathematical calculation method". A procedure for operating the cuff and pressure transducer is described at col. 11, lines 5-65. It is noted that this patent does not provide any specifics as to how and where the transducer is affixed to the patient. At col. 8, lines 32-36 it is stated that the pressure transducer may be that supplied with a known type of blood pressure monitor. As indicated at col. 3, lines 6-23 , this known type of device appears to operate on the oscillometric, as opposed to the tonometric, principle.
U.S. Pat. No. 4,295,471, issued Oct. 20, 1981, entitled "Non-Invasive Vascular Waveform Transducer and Apparatus" describes the use of an inflatable cuff 10 that includes a transducer 30. The transducer is located near a blood vessel to be monitored when the cuff is installed (col. 3, lines 53-62). The transducer 30 is used to detect Korotkoff sounds as the cuff is deflated (col. 4, lines 7-12). A dual sensing approach, described at col. 5, lines 21-40, is used to reject common mode signals (col. 5, line 54 to col. 6, line 16). The determination of MAP is described at col. 14, lines 38-54. A continuous display of an arterial waveform is described at col. 15, line 64 to col. 16, line 12. All embodiments of the transducer 10 appear to be circular in shape (FIGS. 2a, 5a, 5b, 5c), and not shaped to generally conform to the extremity of the patient.
U.S. Pat. No. 5,103,830, issued Apr. 14, 1992, entitled "Electronic Sphygmomanometer", describes an electronic sphygmomanometer that uses both the Korotkoff and the oscillometric methods. A cuff 1 has a Korotkoff sound sensor disposed on a "periphery" of the cuff (col. 6, lines 10-23), and a pressure sensor 21 is connected to the cuff 1.
U.S. Pat. No. 4,653,506, issued Mar. 31, 1987, entitled "Method of Indirect Measurement of Arterial Tension and a Device for Pulse Wave Registration", describes the use of a cuff 3 under which a device 9 is applied to a patient's body. The passing of a blood pulse wave under the device 9 is converted to an electrical signal by a piezo-sensitive cell 17 (col. 6, liens 13-19). Cuff pressure is monitored by a pressure gauge 11. The cuff pressure does not appear to be converted to an electrical signal, and no means is disclosed for correlating signals received from the device 9 with cuff pressure.
U.S. Pat. No. 4,951,679, issued Aug. 28, 1990, entitled "Pulse Wave Detecting Apparatus Having Placement-Condition Detecting Means", describes a tonometric-type device that includes a pulse wave detecting probe 16 that is fixed with a band 14 fastened around a patient's arm. The probe 16 is pressurized with a fluid for urging a diaphragm 22 toward the surface of the arm (col. 3, line 37 to col. 4, line 9).
U.S. Pat. No. 5,033,471, issued Aug. 23, 1991, entitled "Method and Apparatus Measuring Blood Pressure", is also directed to a tonometric-type device that appears to be similar to that described above with respect to U.S. Pat. No. 4,951,679 (see col. 6, line 56 to col. 7, line 55).
U.S. Pat. No. 5,172,696, issued Dec. 22, 1992, entitled "Photoelectric Sphygmomanometer of Volume Oscillometric Method-Type", describes a photoelectric sphygmomanometer of a "volume oscillometric" type. A cuff 1 is fastened to the wrist or fingertip of a patient. A pressure sensor 6 senses cuff pressure. A pulsation sensor 8, located inside the cuff, includes a LED 8a and phototransistor 8b for irradiating arterial blood and detecting light reflected from the arterial blood, respectively (col. 4, lines 10-36). Measurements of cuff pressure and pulsation signals are used to determine systolic, diastolic, and mean blood pressure (col. 8, lines 14-26). The use of a tonometric, under-cuff sensor, that may be shaped to conform to the extremity of the patient, is not disclosed. U.S. Pat. No. 4,924,871, issued May 15, 1990, entitled "Motion Artifact Detection for Continuous Blood Pressure Monitor Transducer", describes a tonometric-type device that includes an array of individual pressure sensitive transducers 22A-22E, all of which are small relative to the diameter of an artery (FIG. 4, col. 3, line 59- col. 4, line 22). A change in pressure within a pressurizable chamber 40 is detected and interpreted to indicate motion of the patient. When motion is detected, data collection is interrupted (col. 5, line 55 to col. 6, line 31).
U.S. Pat. No. 4,860,760, issued Aug. 29, 1989, entitled "Electronic Blood Pressure Meter Incorporating Compensation Function for Systolic and Diastolic Blood Pressure Determinations", describes an oscillometric-type device wherein a cuff 2 is connected to a pressure sensor 11 (col. 10, lines 16-66).
Also of interest is U.S. Pat. No. 3,527,204, issued Sep. 8, 1970, which describes at col. 4, lines 6-10 the use of a transducer 111 which may be a microphone. The microphone is said to be placed under a cuff when blood pressure is measured using an auscultatory method.
Other patents of interest to this invention include the following. A first group of patents are generally related to oscillometric-type determinations: U.S. Pat. Nos. 5,001,187, 4,427,013, 3,903,872 (which provides a cuff-mounted pressure transducer), U.S. Pat. Nos. 4,543,962, 4,349,034, 4,009,709, 4,984,577, 4,995,399 and 4,074,711. A second group of patents are generally related to tonometric-type determinations: U.S. Pat. Nos. 3,880,145, 4,269,193, 4,423,738, 4,987,900, and 5,005,581 (all of which employ multiple sensors). Also of interest are U.S. Pat. Nos. 4,993,422, 3,102,534, 3,123,068, 3,154,067 (which provides a strain gauge), U.S. Pat. Nos. 3,903,873, 3,926,179, 4,185,621, 4,030,484 and 4,307,727.
Also of interest is a pulse rate monitor shown in U.S. Pat. No. 4,489,731, a cuff-less blood pressure measuring device shown in U.S. Pat. No. 4,068,654, and the systems described in the Abstracts of WO 79/00294 and WO 88/03777.
An invasive blood pressure measuring system that is capable of measuring the mean arterial pressure is described in U.S. Pat. No. 3,893,452.
Also of interest is an article entitled "Evaluation of Arterial Tonometry for Noninvasive Continuous Blood Pressure Monitoring During Anesthesia", Anesthesiology, V71, No. 3A, Sep. 1989 (O. Kemmotsu, et al.); an article entitled "Arterial Tonometry: Review and Analysis", J. Biomechanics, Vol. 16, No. 2, pp. 141-152, 1983 (Gary M. Drzewiecki, et al.); an article entitled "The Meaning of the Point of Maximum Oscillations in Cuff Pressure in the Indirect Measurement of Blood Pressure, Part I.", Cardiovascular Research Center Bulletin, Vol. 8, No. 1, July-September, 1969 (J. A. Posey, et al.); an article entitled "Characterization of the Oscillometric Method for Measuring Indirect Blood Pressure", Annals of Biomedical Engineering, Vol. 10, pp. 271-283, 1982 (L. A. Geddes, et al.); an article entitled "The Indirect Measurement of Mean Blood Pressure in the Horse", The Southwestern Veterinarian, pp. 289-293, Summer 1970 (L. A. Geddes, et al.); and an article entitled "A Transducer for the Continuous External Measurement of Arterial Blood Pressure", IEEE Transactions on Bio-Medical Electronics", pp. 73-81, 1963, (G. L. Pressman et al.).